In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique of transcribing a fine pattern on a Si substrate or the like, which is required for forming an integrated circuit having such fine pattern. However, the microsizing of semiconductor devices has been accelerated, and on the other hand, the conventional photolithography method approaches the limit. In the photolithography method, it is said that the resolution limit for a pattern is about ½ of exposure wavelength and is about ¼ of exposure wavelength even if an immersion method is used. Even though the immersion method with an ArF laser (193 nm) is used, it is estimated that the limit is about 45 nm. From this point of view, EUV lithography which is an exposure technique using EUV light having a further shorter wavelength than ArF laser has been considered to be promising as an exposure technique for 45 nm or below. In this description, the EUV light indicates a ray having a wavelength in a soft X-ray region or a vacuum ultraviolet ray region, specifically, a ray having a wavelength of about 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
The EUV light is apt to be absorbed by any substance and the refractive indices of the substance with respect to this wavelength is close to 1, and therefore, the conventional dioptric system such as the photolithography using visible light or ultraviolet light cannot be employed. For this reason, a catoptric system, i.e. a reflective photomask and a mirror are employed in the EUV light lithography.
A mask blank is a laminated member before patterning which is used for producing a photomask. The EUV mask blank has a reflective layer for reflecting EUV light and an absorbing layer for absorbing EUV light which are formed in this order on a substrate such as glass or the like. For the reflective layer, normally used is a multilayered reflective film which is formed by laminating alternately a layer of high refractive index and a layer of low refractive index whereby the reflectance of light is increased when EUV light is irradiated to the layer surface. As the absorbing layer, a material having a high absorbing coefficient to EUV light, specifically, a material having Cr or Ta as major component, is used.
Patent Document 1 describes that a tantalum-boron alloy nitride (TaBN), a tantalum-boron alloy oxide (TaBO) and a tantalum-boron alloy oxynitride (TaBNO) is preferably usable as the material for the absorbing layer since they have a high absorbing coefficient to EUV light and a low reflectance of deep ultraviolet light having a wavelength region (190 nm to 260 nm) of pattern inspection light.
Patent Document 1 describes that the crystalline structure of the absorbing layer is preferably amorphous in order that the surface of the absorbing layer has an excellent smoothness, and in order that the crystalline structure of a TaBN film, TaBO film or TaBNO is amorphous, the B content in this film is preferably between 5 and 25 at % (atomic percentage, the same expression is used in the description). Patent Document 2 describes TaBSiN as an example of the material for the absorbing layer.